1. Field of the Invention
The present invention relates to an objective lens moving apparatus such as used in an optical disk apparatus to record and/or reproduce information.
2. Description of Related Art
FIGS. 1 through 3 show a conventional objective lens moving apparatus in an optical disk apparatus disclosed in Japanese Utility Model Application Laid-Open No. 62-158632. FIG. 1 is its plan view, FIG. 2 is a cross-sectional view at a cross-sectional line II--II in FIG. 1, and FIG. 3 is a magnified view of the principal part of FIG. 2. In these figures numeral 1 indicates a base of the objective lens moving apparatus. A circular cylinder-shaped pole 2 is attached to the base 1 almost perpendicularly to it. Numeral 3 indicates a lens holder holding an objective lens 5 which focuses light spot on an optical disk (not shown in the figure). The lens holder 3 has protrusions 3a and 3a at both its ends and a circular sliding hole 3b passing through its central part to form a bearing part. The optical axis 5a of the objective lens 5 is set to be parallel with the axis of the sliding hole 3b. The pole 2 is loosely fitted into the sliding hole 3b and the lens holder 3 can be freely turned around the pole 2 and freely slid in the axial direction, and it is supported on the base 1 by an elastic member 4 made of silicone rubber in the shape of a rectangular frame. On the lens holder 3 a focus control coil 6 is fixed which moves the objective lens 5 in the direction of the axis 2a of the pole 2 to control the focus of the light spot by moving the lens holder 3. And on the focus control coil 6, a tracking control coil 7 is set which turns the objective lens 5 around the pole 2 to move it in the radial direction of a track so as to control tracking. Permanent magnets 8a for moving which are set on the base 1, upper yokes 8b, and inner yokes 8c which are set on the base 1 inside of the focus control coil 6 make a magnetic circuit intersecting the focus control coil 6 and the tracking control coil 7. The middle parts 4a and 4a of the long sides of the elastic member 4 are shaped slightly flat, and the elastic member 4 is fixed on the upper yoke 8b by means of set screws 9 and 9. Fitting holes 4b and 4b are provided in the middle parts of the short sides of the elastic member 4, and the lens holder 3, whose protrusions 3a and 3a are fitted into these holes 4b and 4b, is supported by the elastic member 4. The set screws 9 and 9 are disposed respectively at locations vertically symmetric with respect to the pole 2, namely, respectively at locations dimension A vertically distant from the horizontal center line (2HL) as shown in the figure.
Next, operation of this mechanism is described below. When deviation of focusing of the light spot has been detected by a detection sensor (not shown in the figure), an electric current corresponding to an amount of this deviation flows into the focus control coil 6 and focusing control of the light spot is performed by moving the lens holder 3, namely, the objective lens 5 in the direction of the axis 2a of the pole 2 under the influence of the magnetic field made by the permanent magnets 8a. At this time the elastic member 4 of silicone rubber controls elastically any excessive movement of the lens holder 3. And, in the case where a deviation of the light spot from a track has been detected, an electric current corresponding to an amount of this deviation flows into the tracking control coil 7 and tracking control is performed in the radial direction of the track by turning the lens holder 3 around the pole 2 under the influence of the magnetic field made by the permanent magnets 8a. In this case also, any excessive movement of the lens holder 3 is controlled elastically by the elastic member 4.
Since a conventional objective lens moving apparatus is composed in the above-mentioned way, it may be sometimes assembled in a state that the optical axis 5a of the objective lens 5 is slanted against the axis 2a of the pole 2 as shown in FIG. 3 when fixing the elastic member 4 by means of the set screws 9 and 9, due to existence of a little clearance between the sliding hole 3b of the lens holder 3 and pole 2, and as a result, this design suffers from the problem that aberration of the focused light spot is often increased.